Cable testing method by transmitting pulses of different rates of propagation



P 1950 J. J. GILBERT CABLE TESTING METHOD BY TRANSMITTING PULSES F DIFFERENT RATES OF PROPAGATION Filed Dec. 10, 1948 Sheets-Sheet 1 3 FIG./ U u -m 5 19.5 S

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//Vl/EN7'OR J. .1 G/LBER T A T TORNE V Sept. 12, 195 J. J. GILBERT 2,522,362

CABLE TESTING METHOD BY TRANSMITTING PULSES OF DIFFERENT RATES OF PROPAGATION RECEIVING TERMINAL Filed Dec. 10, 1948 3 Sheets-Sheet 2 HAND mrsns PASSING F I G 3 l W551i? 0 p I I FREQUENCY fr osc. BAND F/U. 22 PASS/NG- I /3 KC.

I 13 KCM/DBAN 33 osc. I FOR 13 KC. I I 32 MODULATION j REPEA TERE'D T557 24 I susmnmz CABLE 55 KC. BAND F/u: 23 PASS/N6 I I) i I 3/ I a/Kc. mom/v0 26 I o. sc I I 30 29 0/ k6. L

8/ KC. AME 5 1 T0 SWEEP l l TRIGGER PULSE 2/ I I 27 25 CONTROL CIRCUIT I TRANSM/ T TING TERMINAL DELAY Q I 44 5 52 53 AAAA wad/an A T TORNE V Se t. 12, 1950 J. J. GILBERT 2,522,362

CABLE TESTING METHOD BY TRANSMITTING PULSES OF DIFFERENT RATES OF PROPAGATION Filed Dec. 10, 1948 3 Sheets-Sheet 3 ,rzifi ofifm H6. 5 OR 13 KC. 65 6023M I 54 PULSE FIG. 4

l I 50 OR 60 aurpur 74 ADDITIONAL suns 80 AMPLIFIER IF REQUIRED 535 1 3 552 1 RECTIFIER FROM BIKC. J RECEIVING 541m 8? 83 FILTER 8/ 7.975651? comm/t 84 53325 lNl EN7'0R J.J. GIL BERT A T TORNE V Patented Sept. 12, 1950 CABLE TESTING METHOD BY TRANSMIT- TING PULSES OF DIFFERENT RATES OF PROPAGATION John J. .Gilbert, Douglaston, N. Y., assignor to a Bell Telephone Laboratories, Incorporated, New York, N. Y,, a corporation of New York Application December l0, 1948, Serial No. 64,653

This invention relates to a method and means 6 Claims. (01. 179-17531 for testing long, inaccessible electrical cables,

and particularly to a method and means for testing long, submarine cables used for the transmission ofintelligence by telephony or telegraphy.

The object of the invention is a method and means for indicating, at an accessible point in thecable, the presence. of transmission irregularities in an inaccessible length of the cable.

A feature of the invention is a method and means for testing an electrical cable by supplying to the cable a short pulse of electomagnetic waves having one velocity of transmission and, after a predetermined time interval, supplying to the cable a second short pulse of electromagnetic Waves having a highervelocity of transmission, so that the second pulse will overtake and pass the first pulse, causing the two pulses to overlap within a determinable portion of the cable, and measuring the products of the intermodulation of the waves.

Another feature of the invention is means for repeatedly supplying to the .cable successive pulses of electromagnetic waves'having different velocities of transmission, and varying the time intervals between the supply of the successive pulses. i V p In recent years, the use of long cables for the transmission of intelligence, such as telephone and telegraph cables, has been rapidly increasing; and, this use has aroused considerable interest in the problem of usingcables of this general character laid through tellitory, such as through the ocean or other large body of water, in which. the cable is not easily accessible. Due to the transmission losses in such cables, the transmitted waves must be amplified at rather short intervals, thusfnecessitating the use of' submerged re"peater s spaced some thirty to f fty" miles apartalong the cable. 'These repeaters may be encased within an enlargement of the cable sheath; and, with skillful design and good materials, the average useful life of the repeat-- ers is satisfactory, though it is reasonable toexpect that an occasional repeater will have a useful life less than the average. Thus,it is. important to be able to test the electrical condi tion of the individual repeaters from some accessible spot, such as the cable terminals. By such teststhe gradual deterioration in the repeaters may be measured at frequent intervals, and arrangements made to repair a faulty repeater in a season of the year when weather conditions are favorable, materially reducing the cost of the necessary cable ship operations.

The cable and associated equipment are con -l structed to produce a transmission characteristic as nearly linear aspractlcable; and the gradual deterioration of a repeater, or other item of the equipment, produces an irregularity, or non linearity, in the transmission characteristic; When two electromagnetic waves of :difierent frequencies are supplied to a cable system hav: ing a linear transmission characteristic, these.

waves will be transmitted without interaction;

but, if some partof the cable system has a nonlinear transmission characteristic, the two waves will interact to produce intermodulation products, which have theproperties of electromag-. netic waves having frequencies related to the frequencies-of theoriginal waves. a

It has been found that the time of propagation of a sinusoidal current over a long cable depends upon the frequency of the current; the'higher frequency currents being propagated at somewhat faster speeds than the lower frequency currents. If a short train of waves of the lowerfrequency is supplied to the cable, then, after a short time interval, another train of waves of the higherfrequency is supplied to, the cable, due to the difference in the velocities of propa gation, the train of waves of the higher frequency will overtake the other train, the. two trains of waves will overlap in some portion of the cable, the train of waves of the higher frequency, eventually passing the other train .of waves and reaching the end of the cable before the other train of waves. If,'in the portion of the cable in which thetwo trains of waves over-- lap, there is some element, such as .a defective repeater, having a non-linear transmission characteristic, this element will cause the two waves. to intermodulate and to produce intermoduationproducts, having frequencies differing from the frequency ofeither of the trains of waves. These.

intermodulation products will be propagatedto the end of the cable, where these products may be detected to give an indication that the por', tion of the cable has a non-linear characteristic.

By starting with a small time interval, and repeatedly supplying trains of waves separated. by gradually increasing intervals of time, the

successive pairs of trains of waves may be caused to overlap in successive portions of the cable,; until the whole length of the cable has been;

tested.

The invention will be easily understood from" Fig. 3 is a more elaborate cable system embodying the invention; and

Figs. 4, 5, 6 are respectively a timing circuit, oscillator circuit, and control circuit forming parts of the system shown in Fig. 3.

Fig. 1 is a graph of the computed delay, or time taken to propagate electrical waves of different frequencies through a cable of about 2000 nautical miles in length, which might be used for the transmission of telephone and telegraph messages under the Atlantic Ocean. The useful band of frequencies used in the operation of this cable would extend. from say 20 kilocycles per second to say 110 kilocycles per second, and in this region the delay is slightly more than 19 milliseconds, the lowest value being in the vicinity of 80 kilocycles per second. The delay for a wave having a frequency of about 12 kilocycles per second is more than .20 milliseconds. The velocity of propagation of a wave of about 80 kilocycles per second is some 105,000 nautical miles per second, and the velocity of propagation of a wave of 12 kilocycles per .second is some 100,000 nautical miles per second, but, due to the phase shifts in the filters used in the system, the delay time for a wave of 12 kilocycles per second is increased.

The cable system may be of the type shown in United States Patent 2,009,438, July 30, 1935, H. W. Dudley, in which the frequency of one of the oscillators is adjusted to 12 kilocycles per second; and the other oscillators are adjusted to produce channels spaced 4 kilocycles per second apart, from 20 kilocycles per second to .108 kilocycles per second.

In Fig. 2, the commutators l, 2, are secured to shaft 3 and rotated at a fairly high speed by some suitable motor means (not shown). The surfaces of the commutators I, 2, are formed of non-conducting material, except for the narrow conducting segments 4, 5. The brushes 6, I, are insulatingly mounted upon some suitable support (not shown) and bear on the surface of commutator I. The brushes 8, 9, bear on the surface of commutator 2 and are supported by an insulator l afiixed to a gear ll rotatably mounted on shaft 3. A worm I 2, engaging the gear H, 'is mounted on a shaft 13, journalled in some suitable support (not shown) and may be rotated by the knurled knob [4. By rotating the knob 14 the brushes 8, 9, will be traversed around the circumference of the commutator 2.

A local oscillator l5, having a frequency of say 1 kilocycle per second, is connected in serial relationship with the primary windings of two transformers I6, H. The secondary winding of transformer and the brushes 6, 1., are connected to the input circuit of the 12 kilocycles per second channel, so that, when the brushes 5, 1, make contact with the segment 4, a train of waves having a frequency of 13 kilocycles per second will be supplied to the cable. Similarly, the secondary winding of transformer l1, and the brushes 8, 9, are connected to the input circuit of the 80 kilocycles per second channel, so that, when the brushes 8, -0, make contact with the segment 5, a train of waves having a frequency of 81 kilocycles per second will be supplied to the cable.

The trains of waves are transmitted through the sending apparatus SA, of station A, to the cable and through the cable to the receiving apparatus RB, of station B, where the products of intermodulation may be measured; or, as shown, the receiving apparatus RB may be connected to the sending apparatus SE, of another cable at station B, thence through this cable to the receiving apparatus RA, at station A, where, after suitable amplification, as in amplifier IS, the received waves may be registered on some suitable device, such as the cathode ray oscilloscope l9.

For any given speed of rotation of the shaft 3, the lengths of the trains of waves supplied to the cable will be determined by the widths of the segments 5, '5; and the time interval between the supply of the two trains of waves to the cable will be determined by the displacement, around the periphery of commutator 2, of the brushes 8, 9, relative to the brushes 6, 1.

Preferably, the segments 4, 5, are sufliciently wide that one or two complete cycles, at least of each wave are supplied to the cable. These segments should not be too wide, as then the section of cable 'in which overlapping occurs will be undesirably long. It can be shown that the length of cable S, in which overlap of the two trains occurs, is given by where S2 and V2 are respectively the length in nautical miles and the velocity of propagation of the faster train; S1 and V1 are the corresponding quantities for the slower train. Assume the velocity of propagation, V1, of the 13 kilocycles per second wave is 100,000 nautical miles per second, and the velocity of propagation, V2, of the 81 kilocycles per second wave is 105,000 nautical miles per second, then nautical miles For repeaters spaced at intervals of 40 nautical miles along a 2000 nautical mile cable If will vary from 20 microseconds to 1 millisecond. Preferably, the time interval between the first two trains of waves is 20 microseconds, and this interval is increased by 20 microseconds for each successive pair of trains of waves so that the condition of each repeater may be determined.

An alternative arrangement employing electronic timing circuits is shown in block form in Fig. 3. .At the transmitting terminal, a pulse control circuit 2%, of the type shown in Fig. 4, alternately pulses two oscillators 22, 23, of the type shown in Fig. .5, to respectively generate trains of waves having frequencies of 13 kilocycles per second, and 81 kilocycles per second. Since the velocity of propagation of the 13 kilocycles per second wave is less than the velocity of propagation of the other wave, the 13 kilocycles per second oscillator is pulsed first, then, after an adjustable delay, the 81 kilocycles per second oscillator is pulsed. The trains of waves pass through the filters 24, 25, and are transmitted over the repeatered cable 26. The time interval, or delay, between the two trains of waves is adjusted to cause the two trains of waves to overlap in the section of cable containing the repeaters to be tested. If the repeaters in this section of cable are defective, and produce modulation products of substantial amplitude, these products will be propagated, along with the orig inaltrains of waves, to the receiving terminal.

' The 81 kilocycle per second train of waves will be propagated with the largest velocity, and will reach the receiving terminal before the other trains of Waves. At the receiving terminal, the 81 kilocycles per second wave is selected by the band-pass filter 21, amplified and rectified by the amplifier-rectifier 23, which maybe of the type shownin Fig. 6, and supplied to the cathode ray oscilloscope 29, to trigger, or start, the horizontal sweep of the cathode ray. The cathode ray oscilloscope 29 may, of course, be replaced by any other suitable indicating or recording instrument, and the voltage from the rectifier 28 used to initiate the production of one parameter of the record.

Among the products of intermodulation is the difierence frequency fz-2,f1, in the present case 8l-26=55 kilocycles per second, which may conveniently be used to indicate the occurrence of intermodulation. The 55 kilocycles per second wave is selected by the band-pass filter 3|), amplified, if necessary, in the amplifier 3!, and supplied to the vertical plates of the oscilloscope 29. The presence of this component in the received wave will be indicated by a deflection of the 1 cathode ray.

, It is well known that certain types of ampli fiers, such as reverse feedback amplifiers, overload quite abruptly, and this phenomenon may.

justed so that the repeaters in the cable will be,

substantially fully loaded by the two trains of waves, and the amplitudes of this train of waves may be observed on the oscilloscope, so that a sudden change, in amplitude will indicate that a repeater in the section of cable under test has overloaded.

The 13 kilocycles per second wave thus may be selected by the filter 32, amplified, if desired, by amplifier 33 and supplied to the vertical plates of the oscilloscope 29, in place of the outputfrom In Fig. 4, the anode of vacuum tube, 4| is connected through resistor 41 to the grounded anode supply (not shown) and through capacitor C2 to the control grid of vacuum tube 42. The control grid of vacuum tube 42 is connected through adjustable resistor R2 to the anode supply, or, if preferred, to ground. The anode of vacuum tube 42 is connected through resistors 43,v 49, in serial relationship, to the anode supply, and through capacitor C1 to the control grid of vacuum tube 4|. The controlgridof vacuum tube 4| is connected through resistor R1 to the grounded cath odes of vacuum tubes 4|, 42. The heaters of vacuum tubes 4|, 42, are energized from the usual supply (not shown).

Starting with the anode current of vacuum tube 4| cut off, and anode current flowing in vacuum tube 42, the anode of vacuum tube 4| will have a high positive potential, while the anode of vacuum tube 42, and the'junction of resistors 48, 49, will be at a low potential.

A small current will flow through R2 to the control grid of vacuum tube 42 so that this grid will have asmall positive potential maintaining its anode current flowing. A necessary condition to the cutting ofi of anode current in tube 4! is that the control grid of tube 4| be at a negative potential with respect to its cathode. This negative potential diminishes vas the charge'on C1 leaks away to a lower value through R1, the anodecathode path in tube 42 and the resistor 48. It is obvious that when the discharging current in this circuit through R1 approaches zero the potential of the control grid of tube 4| approaches zero causing anode current to flow in this tube. Capacitor C2 had been charged through the gridcathode path of vacuum tube 42 in parallel with R2, the anode supply, and resistor 41. Sincethe high and C2 discharges slowly through R2 and re-- sistor 41. The large negative potential on the control grid of vacuum tube 42 cuts off the flow of anode current in vacuum tube 42. When the anode current of vacuum tube 42 is cut off, the

potential of the anode of this tube, and the point 50, will rise to the potential of the anode supply,- impressing a positive potential on the connection to the point 50. As the capacitor C2 continues to discharge, the discharge current diminishes, reducing the potential drop in resistor R2, and the negative potential on the signal grid of vacuum tube 42, until, eventually, the negative potential is small enough that the anode current can again flow in vacuum tube 42, reducing the potential of the anode, and the connection to point 5|], toa small. value. The duration, of the positive potential on the connection to point 50, which deter mines the length of the train of waves of 13 kilocycles per second supplied to the cable, is determined by the time required to discharge capacitor C2, and this time is controlled mainly by the value of the high resistance of resistor The resistor R2 may be of the order of l megohm, and the capacitor C2 of the order of l 200 micromicrofarads. 3

While tube 4| is conducting, and tube 421s out off, the junction of resistors 48, 49 will have a po-' tential a little less than that of the anode supply, and capacitor 01 will charge up to this higher potential. As the grid-cathode resistance of tube 4| may be fairly low when the grid is driven positive through C1 and the resistance of resistor 49 fairly low, capacitor C1 may be fully charged before capacitor G2 has completed its discharge.

When tube 42 commences to conduct the po-'- tential of the junction point of 4'8, 491falls to a low value, capacitor C1 passes this sudden drop to the control grid of tube 4| driving it negative, cutting ofi the anode current. 01 also commences to discharge through resistor48, the anode-cath- Y ode path of tube 42, and resistor R1. The decaying voltage drop in resistor R1 impresses a de caying negative potential on the signal grid Oi 7. tube 4! until the voltage drop across resistor R1- has fallen to a small value. As the resistance of resistor R1 and the capacitance of capacitor C1 may both be fairly large, the anode current of tube 4! may be cut oli for a number of seconds, or even minutes. As each cycle of operation of this circuit initiates a test of the cable, the operating time of the circuit is the time interval between successive tests.

The anode of vacuum tube 42 is coupled through capacitor C3 to the control grid of vacuum tube 43.. The anode of vacuum tube 43 is connected through resistor 52 to the anode supply (not shown), and through capacitor C4 to the control grid of vacuum tube 44. The anode of vacuum tube 44 is connected through resistor 53 to the anode supply. The cathodes of vacuum tubes 43, 44, are connected together, and through resistor 54 to ground. The anode currents in resistor 54 produce a fairly large positive voltage on the cathodes of vacuum tubes 43, 44. The potential divider, formed of resistors 55, 55, connected from the anode suppl to ground, partially counteracts the bias due to resistor 54, to make the control grid oi vacuum tube 43 normally negative with respect to the cathode so as to cut off, or reduce to a small value, the anode current of tube 4'3. Resistor R4, connected from the control grid of tube 44 to the anode supply, impresses a fairly large positive potential on the control grid of tube 44 which counteracts the bias due to resistor 54, so that the anode current of tube 44 normally is large.

With the anode current of tube 43 cut off, the

point 5| has substantially the potential of the anode supply, thus capacitor C5 is positively charged. When the point 5% is driven positive,

' sending a pulse to control the i3 lzilocycles per second oscillator, this positive potential pulse also passes readily through capacitor C3 and charges capacitor C3 through the grid-cathode impedance of tube '43 in parallel with resistor 56, the anode supply and resistors 48, 49.

Since this positive pulse impresses a positive 7 voltage on the control grid of tube 43, the anode then conducts, lowering the potential of point 5i. When the potential of point 5| is lowered, capacitor 05 will pass this potential drop on to the control grid of tube 45; and capacitor C4 will pass this potential drop on to the control grid of tube 44, cutting oil the anode current in this tube. The anodecurrent of tube 44 is larger than the anode current of tube 43, thus, when the anode current of tube 44 is cut on", the resultant current in resistor 54 is reduced, reducing the voltage on the cathodes of tubes 43, 44, so that tube 43 continues to conduct anode current. The anode current of tube 44 will be cut off until the negative voltage, due to the discharge of capacitor C4 through resistor R4, decreases to a value which is counteracted by the positive potential from the anode supply. This time interval is largely determined by the time constant C4R4, and may be changed by adjusting R When anode current again flows in tube 44, this current flowing in resistor 54 restores the negative bias on the control grid of tube 43. When tube 42 also conducts and capacitor C3 discharges through the anode-cathode path of tube 42 and resistor 55 impressing an additional negative potential on the control grid of tube 43 this has no effect. Tube 43 is thus cut ofi, and the potential of point 5] rises to substantially the potential of the anode supply, causing capacitor C to charge quickly through grid-cathode impedance in parallel with 8 resistor R resistor '52, Clreadily'passing the posi--' tive potential pulse on to the signal grid of tube 44, maintaining the flow of anode current in this tube, and also pass through capacitor C5 a positive potential on to the signal grid of tube 45.

The anode of tube 45 is connected through resistor 59 to the anode supply (not shown) and through capacitor C6 to the control grid of tube 46. The anode of tube 46 is connected through resistor 6! to the anode supply; and the cathodes of tubes 45, 46 are connected together, and through resistor 62, to ground. The current flowing in resistor 62 impresses a fairly large positive voltage in the cathodes of tubes 45, 46. The control grid of tube 45 is connected through resistor R6 to the anode supply, and this positive potential counter-' acts the bias due to resistor 62, so that the anode current of tube 46 is normally quite large. The

potential divider formed of resistors 57, 58, connected across the anode supply and to the control grid of tube 45 supplies a potential which partially counteracts the bias from resistor 62, but not completely, so that the anode current of tube 45 is normally cut off. The heaters of tubes 45, 46, are energized from the usual supply (not shown).

When capacitor C5 passes a negative pulse from point 5i on to the grid of tube 45 because tube 43 starts to conduct, as tube 45 is normally biased beyond out off, this voltage will have no eilect. However, when the discharge cycle of capacitor C4. is completed, and point 5| suddenly rises to a high positive potential, capacitor C5 passes a positive voltage on to the grid of tube 45, causing tube 45 to conduct, thus lowering the potential of the anode of tube 45. Capacitor C6 is connected, through resistors 59, R6, to the anode supply, and is normally charged; but, when the anode potential of tube 45 suddenly falls, capacitor C6 passes this negative pulse on to the grid of tube 46, cutting ofi the flow of anode current in tube 46. Also capacitor C6 will discharge through the resistor 59 and resistor R6. As the anode current of tube 46 is larger than the anode current of tube 45, the cessation of the anode current from tube 46 reduces the bias impressed on the grid of tube 45 by the voltage drop in resistor 62, so that the anode current continues to flow in tube 45 after the charging cycle of capacitor C5 has been completed. The cessation of anode current causes the potential of the anode of tube 43, and of point 60, to rise suddenly, impressing a positive potential on the connection to point After a time interval determined largely by the time constant CcRs, the discharging current of capacitor Cs will decrease until the negative potential impressed upon the grid of tube 46, by the voltage drop in resistor R6, is less than the positive potential from the anode supply, and tube 46 again conducts anode current, lowering the anode voltage and the potential applied to the connection Bil. This heavy anode current, flowing in resistor 62, produces a large bias on the grid of tube 45, cutting off the anode current of tube 45, and causing the anode of tube 45 to rise to substantially the potential of the anode supply. Capacitcr C6 charges quickly through the grid-cathode impedance and Rs in parallel, resistor 62, the anode supply and resistor 59 and also passes on a positive potential to the signal grid of tube 46. The flow of anode current in tube 46 is maintained by the large positive grid bias through R5.

The time constant CzRz controls the length of the train of waves of 13 kilocycles per second; the time constant C4R4 controls the time interval between the trains of waves; the time constant CsRs controls the length the train of waves ,of 81 kilocycles per second; and the time constant C1R1 controls the time interval between successive complete testing cycles. The time interval between the trains of waves may be changed by adjusting B4. In a complete series of tests, this time interval may initially be quite small, and may be increased for each repetition of the test cycle, so that the successive overlaps I of the trains of waves will occur in successive sections of cable until the whole cable has been tested.

Fig. 5 shows a controlled oscillator, suitable for the generation of a train of waves having a frequency of 13 or 81 kilocycle's per second. The

anode of the pentode vacuum tube 64 is'connected through the winding 65, and resistor 61, to the anode supply (not shown). The capacitor $6 is connected across the winding 65 to Iorm a tank circuit tuned to 13, or 81, kilocycles per second.

put winding H is inductively coupled to the winding 65 and is connected through the appropriate filter to the cable. The brush of potentiometer I5 is adjusted to bias the grid so that; the circuit is normally in a non-oscillating condition. I

Thecapacitor M is connected through resistor 13 to the signal grid of tube 64 and to'connection 50, or connection 60, Fig. 4. I I I I When a positive potential is applied by connection 50, or connection 60, to capacitor I4, Fig. 5, capacitor 14 will pass the positive'pulse to the control grid of tube 64 and this positive pulse counteracting the negative bias applied to the signal grid of tube 64 will cause the circuit to oscillate. When the positive potential is cut off by a negative pulse from point orl60 the negative bias is restored to the signal grid of tube 64 and stops the oscillations. The time constant of the resistors 13 and T2 and capacitQr H and other minor elements must be sufficiently long to maintain the.:full time of the positive pulse on the control grid of tube 64. I I In Fig; 6, the 81 kilocycles per second receiving band filter is connected through transformer 80 to a suitable amplifier, symbolically represented by the amplifier tube 8|, and tuned interstage network 82. The amplifier is coupled by capacitor 83 to the conventional voltagedoubling rectifier, formed by rectifiers 84, 85 and capacitors 86, 81 connected to the load resistor 88. The load resistor 88 is connected to the trigger circuit of the oscilloscope to supply a voltage'of the proper amplitude and polarity to start the horizontal sweep of the cathode ray beam.

What is claimed is: I I 4 1. The method of testing an electrical cable for non-linearities in the transmission characteristic of the cable which comprises momentarily supplying to the cable a first electromagnetic wave ofone velocity of propagation, then, after the lapse of a predetermined time interval, momentarily supplying to the cable a second electromagnetic wave of a second, higher velocity of propagation, whereby said waves are caused to overlap within a determinable length of the cable and measuring the magnitudeci the products of the intermodulation of said waves in said length of cable to indicate non-linearities in the transmission characteristic'pf the length of cable. I

2. The method of testing an electrical cable for non-linearities in the transmission characteristic of the cable which comprises momentarily supplying to the cable a first electromagnetic wave of one velocity of propagation, then, after the lapse of a predetermined time interval, momentarily supplying to the cable a second electromagnetic wave of a second, higher velocity of propagation,

whereby said waves are caused to overlap within a determinable length'of the cable, measuring the magnitude of the products of the intermodulation of said waves. in said length of cable to indicate non-linearities in the transmission characteristics and changing said time interval in successive tests to cause the waves to overlap within successive lengths of the cable.

3. Means for testing an electrical cable containing spaced amplifying devices including a first source of electromagnetic waves having one velocity of propagation, means for momentarily connecting said source to the cable to supply a first train of a few cycles of said waves, a second I source of electromagnetic waves having a second, higher velocity of propagation, means for, after the lapse of a predetermined time interval, m0- mentarily connecting said second source to the cable to supply a second train of several cycles of said waves, whereby said trains are caused to overlap in a length of the cable containing some of said devices and to intermodulate in any of these devices which have deteriorated, and means connected to said cable to indicate the products of intermodulation of. said waves due to deterioration of said devices.

4. The combination of claim 3 with means for changing said time interval to cause successive trains of waves to overlap within successive lengths of the cable.

5. Means for testing an electrical cable containing spaced amplifying devices including a first source of electromagnetic waves having one first train of a few cycles of said waves, a second source of electromagnetic waves having a second,

higher velocity of propagation, means for, after the lapse of a predetermined time interval, momentarily connecting said second source to the cable to supply a second train of several cycles of said waves, whereby said trains of waves are caused to overlap in a length of the cable containing some of said devices and to intermodulate in any of these devices which have deteriorated, means for recurrently supplying trains of said waves to the cable and means connected to the cable to indicate the products of intermodulation of said waves.

6. The combination in claim 5 with means for changing said time interval between the recurrent applications of said waves.

JOHN J. GILBERT.

REFERENCES I CITED The following references are of record intho file of this patent: 

